There is a need to extract the maximum possible performance from a Coriolis type angular velocity sensor, also known as a Coriolis type gyroscope, whilst maintaining low costs to construct the angular velocity sensor and maintaining small size characteristics of the sensor.
The device described in International patent publication number WO2006006597 is an example of a Coriolis type gyroscope formed using MEMS techniques and which is designed to meet the performance requirements of a high volume automotive application. This device operates using a cos 2θ vibration mode pair as described with reference to FIGS. 1a and 1b of WO2006006597, reproduced herein as FIGS. 1a and 1b. In operation, one of these vibration modes is excited as a primary carrier mode as indicated by axis P, the extremities of which are illustrated in FIG. 1a as dashed lines. When the gyroscope is rotated around an axis normal to the plane of a planar silicon ring that forms the gyroscope, Coriolis forces are generated which couple energy into the other vibration mode as indicated by axis S, that is a secondary response mode, the extremities of which are illustrated in FIG. 1b as dashed lines. The amplitude of the induced motion in the secondary response mode will be directly proportional to the applied rotation rate which acts on the gyroscope.
Such a gyroscope will typically be operated in a closed loop mode. In this mode, the primary carrier mode P is driven at the resonance maximum by a primary drive transducer connected to a Phase Locked Loop and the amplitude of motion is substantially maintained at a constant value using an associated Automatic Gain Control loop. The Automatic Gain Control loop is arranged to compare the amplitude of motion, as measured at a primary pick-off transducer, to a fixed reference level, and to dynamically adjust the drive level of the primary drive transducer to maintain a constant signal level and hence a constant amplitude of motion.
The Coriolis force will induce motion in the secondary response mode S, which is detected using a secondary pick-off transducer and in a closed loop operating mode such motion is substantially nulled by means of a secondary drive transducer. It will be understood that the value of the drive force generated by the secondary drive transducer to maintain the null is a direct indication of the applied rotation rate due to Coriolis force acting on the gyroscope.
Devices utilising planar silicon ring structures, typically require that the cos 2θ vibration mode frequencies, i.e. the frequencies of the primary carrier mode and the secondary response mode, are accurately matched. This gives maximum sensitivity as the motion induced by a Coriolis force is amplified by the quality factor of the secondary response mode. Amplification by the quality factor of the secondary response mode can be of the order of several thousands. MEMS fabrication processes are capable of producing planar silicon ring structures to a high degree of accuracy. However, small imperfections in the geometry of such a structure will typically give rise to a small residual frequency split between the primary carrier and secondary response mode frequencies. For the device described in International patent publication number WO2006006597, this frequency split can be compensated for during operation of the device by the use of additional transducers, similar in construction to the drive transducers and pick-off transducers that are arranged externally to the planar ring structure. Each additional transducer is positioned internally of the planar ring structure. Accordingly, when a direct current (DC) signal offset is applied between a capacitor plate of an additional transducer and the planar silicon ring, an electrostatic force is generated that acts as a negative spring and allows the stiffness of the planar silicon ring to be locally adjusted. Therefore, the vibration mode frequencies can be differentially adjusted using such additional transducers to ensure that the vibration mode frequencies are accurately matched.
Referring to FIG. 2, a gyroscope structure 1 as described in International patent publication number WO2006006597 has a ring structure 2 supported from a central hub 3 by eight compliant support legs 4a to 4h. Drive transducers 5a, 5b, 6a and 6b and pick-off transducers 7a, 7b, 8a and 8b are all located around the outer circumference of the ring structure 2 and are each spaced from the ring structure 2 to create a capacitive gap. In closed loop operation, two opposed primary drive transducers 5a and 5b are used to excite the primary motion of the ring structure 2. Excited primary motion is detected by two opposed primary pick-off transducers 7a and 7b. Coriolis induced motion of the ring structure 2 is detected using two opposed secondary pick-off transducers 8a and 8b and such Coriolis induced motion is nulled using two opposed secondary drive transducers 6a and 6b. The gyroscope structure 1 includes sixteen capacitor plates 9a to 9p that are all located within the ring structure 2 and are each spaced from the ring structure 2 to create a capacitive gap. Each capacitor plate 9a to 9p is arranged to generate a predetermined electrostatic force that acts upon the ring structure 2 to locally adjust the stiffness of the ring structure 2.
Referring to FIG. 3, in which like references have been used to indicate similar integers to those illustrated in FIG. 2, the ring structure 2 of the gyroscope structure 1 is formed in a layer 10, which is fabricated from bulk crystalline silicon. The ring structure 2 is supported from the central hub 3 and the central hub 3 is anodically bonded to glass support layers 11 and 12 adjacent to layer 10. Glass support layers 11 and 12 are typically fabricated from Pyrex®. Capacitor plates 9h and 9p, which are also fabricated from crystalline silicon, are directly bonded to the glass support layer 11. Support legs 4d and 4h support the ring structure 2 from the central hub 3.
This glass to silicon to glass sandwich formation provides a hermetic device and is assembled with a low pressure gas, for example 10 mTorr (1.33322 Pa), within the spaces defined by layer 10 and one or more cavity regions 13 within the glass support layers 11 and 12 to enable the ring structure 2 to resonate with a high quality factor, typically in the order of 10,000 to 100,000. Silicon material is used to manufacture the ring structure 2, central hub 3, support legs 4a to 4h, drive transducers 5a, 5b, 6a and 6b, pick-off transducers 7a, 7b, 8a and 8b and capacitor plates 9a to 9p as it is inexpensive, readily available and enables simple etching to fabricate such components.
The amplitude of motion of the ring structure 2 is set to be several microns. Accordingly, in operation, the annular ring structure 2 forms successive prolate and oblate elliptical shapes along the drive direction.
Referring to FIG. 4, a primary control loop for a sensor comprising a resonant structure, as described in European patent publication number EP0565384, includes a primary control loop arranged between a primary pick-off electrode 18 and a primary drive electrode 14 and is used to drive the resonant structure in a primary mode at the resonant frequency of the resonant structure. A voltage controlled oscillator 34 is used to drive the resonant structure and a gain control 48 allows the amplitude of the drive signal applied to drive electrode 14 to be varied in response to a control signal generated by a controller 46. The output of the primary pick-off 18 is demodulated by demodulator 30 of a frequency loop portion of the primary control loop using the voltage controlled oscillator 34 output frequency as a reference, but phase shifted by 90°. A loop filter F(s) 32 is arranged to provide closed loop stability at the operating point of the resonant structure and the output of the loop filter F(s) 32 is connected to the voltage controlled oscillator 34. A second demodulator 44 of a gain control loop portion of the primary control loop utilizes the output of the voltage controlled oscillator 34 as a reference to demodulate the output of the primary pick-off 18. The demodulated output of demodulator 44 is fed to the controller 46 which is arranged to compare the demodulated output with a reference voltage Vref. The difference between the demodulated output from the demodulator 44 and the reference voltage Vref is used as an error signal which is feed to the gain control 48. It will be understood that the output of the controller 46, the error signal, sets the amplitude of motion generated by the primary drive electrode 14 so as to maintain the primary pick-off signal from the primary pick-off electrode 18 at a fixed level, as set by the value of the reference voltage Vref. Thus the primary motion of the resonant structure is set by two parts of the primary control loop, first the frequency loop of the primary control loop sets the primary pick-off signal to be 90° out of phase with the voltage controlled oscillator 34 and thereby sets the resonant structure to operate at resonance and second the gain control loop of the primary control loop sets the primary pick-off signal to a fixed value to stabilize the amplitude of motion of the resonant structure.
A secondary control loop is used to sense a secondary mode at the primary resonant frequency of the resonant structure. The secondary mode is due to motion of the resonant structure under the influence of angular rotation of the resonant structure, i.e. the Coriolis effect. Such motion is detected at 45° with respect to the primary mode of the resonant structure and is proportional to the applied rate of rotation. The secondary control loop is arranged between a secondary pick-off electrode 20 and a secondary drive electrode 16 and generates an applied force feedback at the resonant frequency to null the secondary mode. Accordingly, the amplitude of a signal applied to the secondary drive electrode 16 will be proportional to the applied rate of rotation of the resonant structure. A benefit of the secondary control loop is that the bandwidth of the sensor is set by the closed secondary control loop rather than by the quality factor of the resonant structure.
The secondary control loop includes two demodulators, a first demodulator 54 with a quadrature phase reference input and a second demodulator 56 with an in-phase reference input. Each demodulators 54, 56 is arranged to demodulate secondary pick-off signals received from the secondary pick-off electrode 20 with its respective reference input and to produce demodulated outputs. The demodulated outputs of the demodulators 54 and 56 are feed into regulators G(s) 58 and 60, respectively. Regulators G(s) 58 and 60 are arranged to maintain the stability of the closed secondary loop and produce outputs which are then modulated by modulators 62 and 64 respectively. Modulator 62 has an in-phase reference input and modulator 64 has a quadrature phase reference input. Each modulator 62 and 64 is arranged to modulate the outputs of the regulators G(s) 58 and 60 with its respective reference input and to produce modulated outputs. The combined modulated outputs of modulators 62 and 64 supply a signal that is applied to the secondary drive electrode 16 to null motion detected by the secondary pick-off electrode 20.
Accordingly, the secondary control loop is formed of two parts, a quadrature-phase loop comprising demodulator 56, regulator G(s) 60 and modulator 64 and an in-phase loop comprising demodulator 54, regulator G(s) 58 and modulator 62. A quadrature signal arises due to the difference between the primary and secondary modes and is not rate sensitive. Whereas, the in-phase signal is a rate sensitive signal. Thus the signal applied to the secondary drive transducer 16 has both in-phase and quadrature components.
The rate of rotation applied to the resonant structure is determined by demodulating the combined modulated output for the modulators 62 and 64 in demodulator 26 with the same reference signal derived from the voltage controlled oscillator 34 of the primary loop phase shifted by 90°.
As illustrated in FIG. 4, the primary pick-off electrode and the secondary pick-off electrode are coupled to the primary control loop and the secondary control loop, respectively, via charge amplifiers. A high pass filter 52 is used to suppress noise in the secondary control loop, which requires a high level of gain to achieve a good level of sensitivity.
However, the prior art as typified by that described with reference to FIGS. 1a to 4 suffers from a charge trapping effect.
The scale factor of a capacitive type ring gyroscope depends on the 4th power of the gap dE defined between a ring and its associated drive and pick-off transducers, the scale factor SF being expressed as:
  SF  ∝            1                        G          SD                ⁢                  G          PPO                      ⁢          1              HT        2              ⁢                  d        E        4                              (                                    ɛ              0                        ⁢            A                    )                2              ⁢          C      f        ⁢          V      AGC      
Accordingly, the scale factor SF varies with the inverse square of an applied DC voltage HT to the ring as well as the fourth power of the gap dE. The scale factor SF is defined as the secondary drive voltage required to null a particular rate in degrees per second. Cf is the feedback capacitance of a charge amplifier, A is the area of a capacitor plate, ∈0 is the permitivity of free space and VAGC is the voltage supplied by an automatic gain controller. GSD and GPPO are the gains of the secondary drive and primary pick-off transducers.
A typical capacitor gap dE is approximately 10 micrometres for a ring diameter of 8 millimeters and a typical DC voltage HT for a high performance ring gyroscope is about 50 volts. At such a DC voltage HT a phenomenon of charge trapping occurs. This manifests itself as a reduction in the scale factor SF and can amount to approximately a 1% reduction in scale factor SF over a period of days if a gyroscope is maintained at a temperature of 85° C. from switch on. A reduction in scale factor SF implies that the effective gap dE has reduced and/or the DC voltage HT has increased. A high performance ring gyroscope needs to provide a scale factor SF accuracy of better than 0.1% to be of commercial value. Accordingly, the charge trapping phenomenon gives rise to an unacceptable error. The effect of charge trapping can be thought of as either a change in the gap dE, and/or a change in the effective DC voltage HT. For example, a change in the scale factor SF of 1% corresponds to a gap dE change of 25 nanometres.
It is also known that switching the DC voltage HT off, and leaving the gyroscope at a temperature of 85° C. over a period of time, resets the charge trapping effect. Thus there is hysteresis effect in the scale factor SF due to charge trapping that makes it difficult to apply compensation, as such compensation will be history dependent.
Also, the offset bias of the ring gyroscope needs to be better than 10 degrees per hour for a high performance tactical grade device. There is an offset bias change which accompanies the scale factor SF change, so stabilisation of the scale factor SF will also benefit offset bias stability of the ring gyroscope. The offset bias stability can be thought of as being due to the anisotropy of the scale factor SF change. Accordingly, compensation for such changes in offset bias is difficult to achieve.
The inventor has been appreciate that the charge trapping effect is due to impurity trapping sites within the silicon lattice used to form the ring gyroscope. For a MEMS type gyroscope it is usual to employ highly doped high purity silicon. The high level of doping provides a low resistivity of typically 0.03 ohms per centimeter. Under the application of a DC voltage HT, a depletion zone is created near to the surface of the gap dE. The depletion zone occurs very rapidly after application of the DC voltage HT, and is related to establishing a new state of equilibrium. Minority species with the silicon can be swept into this depletion zone and trapped at impurity sites. This has the effect of reducing the effective gap dE. The rate at which this occurs is dependent on the DC voltage HT and the magnitude of the change is dependent on the impurity level at the impurity sites.
An Automatic Gain Controller (AGC) is arranged to give a fixed level of primary pick-off voltage, as discussed above with reference to FIG. 4. The amplitude of motion of the ring is related to the primary pick-off voltage through the transducer gain of the pick-off transducer. If the transducer gain changes, then the amplitude of motion changes even with a fixed AGC set level. The transducer gain in this case is set by the capacitance of gap dE and the DC voltage HT. Thus if the capacitance changes, by a change in the effective gap dE, the amplitude of motion changes. As the scale factor SF of the gyroscope is proportional to the amplitude of motion, changes in the amplitude of motion will cause a change in scale factor SF.
Accordingly, charge trapping is an important error for capacitive based sensors, including ring gyroscopes, that rely on a DC bias voltage HT. The DC bias voltage HT has the benefit of increasing the gain of both the drive and pick-off transducers and to linearise the response. Thus the output force is proportional to an applied alternating current (AC) signal in the presence of the DC bias, where normally the electrostatic force is a quadratic function of the AC signal. Also the pick-off signal appears at the resonance frequency.
Schemes which do not use a DC bias are possible, such as AC biasing, but this is a more complicated scheme which is difficult to implement. The AC bias is a means of eliminating the effects of charge trapping as there is never a steady field in which charge carriers can drift. One example of AC biasing is described in U.S. Pat. No. 5,760,304, in which the DC and AC signals are replaced by two AC signals at different frequencies, such that the difference frequency equals the resonant ring frequency.
Alternatively, a lower voltage scheme as described in International patent publication number WO0122094 can be used with a comb type drive transducer rather than the parallel plate capacitor type drive transducer of the ring gyroscope described with reference to FIGS. 2, 3 and 4. However, it is difficult to implement comb type drive transducers and/or pick-off transducers with a ring type gyroscope due to the presence of both radial and tangential motion of the ring.